Using Power Flags / optional components

Schematic
1

As I understand it, a “power flag” is useful if the power is coming via a generic connector, and you wish to tell ERC “actually, this is a power connection”… is that a good description?

On my PCB, I need to lay out two +5v to +/-12v DC:DC converters: they are actually very similar but have a slightly different pinout, but only one is ever fitted (so I have a note on the PCB “only fit one of these!”).

In addition, I have 2 possibilities of the USB connector for the incoming +5v supply: one is an actual USB pcb socket, the other is a small “USB connector Module” (which connects via 5 x 0.1" molex pins), again, only one is fitted

Over on the schematic, I clearly need to have the 2 different USB sockets, and 2 different DC:DC converters shown (they are both “wired in parallel”), but clearly ERC does not like this: I have 2 sources of +5v (from the 2 USB sockets) and 2 sources of the +12v and -12v (from the 2 DC:DC converters)! I get loads of errors, related not only to the above components, but for other components, but I’m guessing that they are all coming from the fact that I have 2 power sources (twice over!)

What is the suggested way to fix the problem, and keep ERC happy? The Power Flag doesn’t seem to fit the bill, so do I need to go and just edit the relevant components, and make the power sources “passive”, or is there a better way?

(I should add that, yes, if I simply ignore the ERC errors, all actually works fine: on the PCB, it does connect the components as desired - but it would just be nice to clean up ERC, so I don’t miss a “real” error!)

2

Indeed, that is the Idea of the PWR_FLAG symbols.

For the rest, I assume your DC:DC converters are some kind of fixed module and not built out of discrete components. If they were built out of discrete components, they would probably have a diode in the power line, which would break the ERC check for the Power-Output pins. And so do passive filters with inductors, ferrite beads or other components in series with the power supply, and that is another use for the PWR_FLAG symbols.

Setting the electrical output to “Passive” for only one of the DC:DC converters would make ERC happy, but would make your schematic “asymmetrical”.

Another way is to use a “Net-Tie” between the nets. On the PCB a “Net-Tie” consists of 2 (or more) pads connected by copper, so it would add a little bit of nuisance for the PCB layout.

Ignoring the ERC errors would not be my preference. Especially when you start to ignore more ERC errors, then it becomes difficult to distinguish between the real and the false positives.

I would set the outputs of both DC:DC converters to “passive” and then add a PWR_FLAG near the place where the outputs of these 2 DC:DC converters come together.

An alternative is to design a custom footprint for the DC:DC converter, that overlays the footprints of different modules, so you can choose later which power module you put into the PCB.

So what are those other ERC errors? How many disappear if you fix the outputs of the DC:DC converters?

3

Thanks for those thoughts, very useful, and kinda mirrored the way I was thinking.

The problem of having a custom footprint for both DC:DC converters in one is two-fold: first, the footprints are actually quite similar, but would “clash”:
image
or

image.

Second, as you can see, I tend to fit the two devices differently on different boards (depending on how tight for space I am!)

I actually have the 4 different components (2 x USB connector, 2 x DC:DC converter) in my own library, so editing them will be easy.

You asked about the errors:

ErrType(4): Conflict problem between pins. Severity: warning
@(4.925 in, 9.425 in): Pin 6 (Power output) of component U7 is connected to
@(15.350 in, 1.575 in): pin 1 (Bidirectional) of component J1 (net 3).
ErrType(4): Conflict problem between pins. Severity: warning
@(15.350 in, 1.575 in): Pin 1 (Bidirectional) of component J1 is connected to
@(4.925 in, 10.425 in): pin 6 (Power output) of component U8 (net 3).
ErrType(4): Conflict problem between pins. Severity: warning
@(4.925 in, 10.425 in): Pin 6 (Power output) of component U8 is connected to
@(7.835 in, 10.425 in): pin 1 (Bidirectional) of component J24 (net 3).
ErrType(5): Conflict problem between pins. Severity: error
@(4.925 in, 10.650 in): Pin 5 (Power output) of component U8 is connected to
@(4.925 in, 9.650 in): pin 5 (Power output) of component U7 (net 4).
ErrType(4): Conflict problem between pins. Severity: warning
@(4.925 in, 9.650 in): Pin 5 (Power output) of component U7 is connected to
@(7.835 in, 10.850 in): pin 1 (Bidirectional) of component J25 (net 4).
ErrType(4): Conflict problem between pins. Severity: warning
@(4.925 in, 9.200 in): Pin 7 (Power output) of component U7 is connected to
@(7.835 in, 10.000 in): pin 1 (Bidirectional) of component J22 (net 5).
ErrType(4): Conflict problem between pins. Severity: warning
@(7.835 in, 10.000 in): Pin 1 (Bidirectional) of component J22 is connected to
@(4.925 in, 10.200 in): pin 7 (Power output) of component U8 (net 5).
ErrType(4): Conflict problem between pins. Severity: warning
@(1.300 in, 10.300 in): Pin 5 (Power output) of component Module1 is connected to
@(2.285 in, 9.715 in): pin 1 (Bidirectional) of component J19 (net 47).
ErrType(4): Conflict problem between pins. Severity: warning
@(1.000 in, 10.900 in): Pin 1 (Power output) of component Module1 is connected to
@(2.285 in, 10.075 in): pin 1 (Bidirectional) of component J23 (net 66).

(J7 & J8 are the 2 DC:DC converters, Module 1 & J18 are the two USB connectors. J19-23 are just 1-pin molex connectors).

I suspect if I make all power things be “passive” and use the “power flags” as you described, then the above will probably clear up… I’ll let you know!!!

4

All the ERC errors are between a “Power output” and a “Bidirectional” pin.

If you want ERC to be correct, you need the “Power Output” pin type for either the output of your DC:DC converter, or via a PWR_FLAG symbol on the same net, or else you will get errors for the power input pins of the IC’s that are powered by your DC:DC converters.

It is unusual to set connector pin types to bidirectional. Normally connector pins are set to “passive” and this will (almost) never cause an ERC error.
See also the ERC matrix:
Eeschema / Inspect / ERC / Options (tab)

image
image709×700 58.1 KB

5

If it’s not a module with integrated inductor, even properly configured Buck converter with external inductor will break the rule.

6

(I think the reason for the USB connectors having pins set wrongly was that I created them in my early days of playing with KiCAD, and knew no better!!! I’m more experienced now, so understand better!)

Ok, so I just set all the power pins of the DC:DC converters, and the USB connectors to be passive, and added in the Power flags, and all those warnings are gone.

All I have left now are 2 warnings, which I can live with:

ErrType(4): Conflict problem between pins. Severity: warning
@(7.275 in, 7.175 in): Pin 15 (Bidirectional) of component U6 is connected to
@(4.525 in, 9.775 in): pin 1 (Power output) of component #FLG0104 (net 14).
ErrType(4): Conflict problem between pins. Severity: warning
@(4.525 in, 10.000 in): Pin 1 (Power output) of component #FLG0105 is connected to
@(8.875 in, 8.875 in): pin 6 (Bidirectional) of component U6 (net 22).

These relate to bi-directional pins on a PIC that are held up or down (although, funnily enough, it didn’t complain about another bi-directional pin that’s also held to +5v!!)

Thanks to you for your great advice / help!

7

The DC:DC converters are indeed small modules, such as the XP Power IA0512S (https://uk.farnell.com/xp-power/ia0512s/converter-dc-to-dc-5v-to-12v-1w/dp/8727503)

Many of my circuits are analogue, and need a small amount of +/- 12v, so using a 5v to dual 12v converter works nicely, lets me power them from readily available USB power! These DC:DC converters are small, cheap, and provide more than enough power.

8

Yes, this type of converter module should have symbol properly configured and not need an external Power flag symbol attached.
I mostly use converters with external inductors and few other parts, so in my case I must use PWR_FLAGS more extensively

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